The present invention relates to electrochemical conversion cells, commonly referred to as fuel cells, which produce electrical energy by processing first and second reactants, e.g., through oxidation and reduction of hydrogen and oxygen. By way of illustration and not limitation, a typical cell comprises a polymer membrane (e.g., a proton exchange membrane) that is positioned between a pair of gas diffusion media layers and catalyst layers. A cathode plate and an anode plate are positioned at the outermost sides adjacent the gas diffusion media layers. All of theses components collectively form the cell unit.
The voltage provided by a single cell unit is typically too small for useful application. Accordingly, a plurality of cells are typically arranged and connected consecutively in a “stack” to increase the electrical output of the electrochemical conversion assembly or fuel cell. In this arrangement, two adjacent cell units can share a common polar plate, which serves as the anode and the cathode for the two adjacent cell units it connects in series. Such a plate is commonly referred to as a bipolar plate and typically includes a flow field defined therein to enhance-the delivery of reactants and coolant to the associated cells.
Bipolar plates for fuel cells are typically required to be electrochemically stable, electrically conductive, and inexpensive. Metallic bipolar plates are advantageous because they can be made very thin (e.g., <0.25 mm) and can be formed into a final shape by inexpensive metal forming techniques, such as stamping. However, metal plates are susceptible to corrosion. An active corrosion process in a fuel cell stack can increase the membrane resistance and the contact resistance of the bipolar plates, reducing the power density of the stack. Stainless steels have been considered for use in forming bipolar plates, due primarily to their inherent corrosion resistance and the relatively inexpensive material cost. Accordingly, the present inventors have recognized a need to provide for improved schemes for enabling the use of stainless steels in forming bipolar plates.